I see them lowering their heads into rucks or tackles. While there are fewer hits of course (in football, obstruction/blocking is encouraged off the ball) I think, anecdotally based on watching a lot of both, that rugby players and trainers don't account for concussion incidents as well. You see in both NFL and rugby league/union players shrugging off trainers attempting to take them out of a game, but I feel like I see many more rugby players returning to action after obvious grogginess akin to what I saw in the NFL a decade ago. There is under reporting in both sports too of course.